In the field of optical telecommunication, optical couplers (e.g. MMI couplers) are commonly used as the optical splitters and recombiners in optical circuits such as Mach-Zehnder modulators (MZMs). Normally input and output waveguides are provided for guiding optical signals through the couplers.
FIG. 1a is a schematic illustration of a 2×2 MMI coupler in plan view. As is well known, for a specific design of MMI the MMI length and waveguide pitch (centre-to-centre spacing) of the input 12 and output waveguides 4 are necessarily related to the width of the MMI.
FIG. 1b is a schematic illustration of an idealised input/output ridge waveguide of a 2×2 MMI coupler shown in vertical cross-section. This idealised rectangular waveguide ridge profile is generally not practically achievable at reduced ridge dimensions The input waveguides 12 are formed on a semiconductor substrate comprising a lower confinement (cladding) layer 1, a waveguide core layer 2 deposited on the lower confinement layer 1 and an upper confinement (cladding) layer 3 deposited on the waveguide core layer 2. One or more of these layers is selectively etched, over predetermined widths, W, to define ridges which form the waveguides 12. The etching process also defines an etched gap 5 between two waveguides 12, which controls the profile of the waveguide structure.
In an optical circuit it is advantageous to miniaturize the size of the MMI in order to provide a more compact circuit. In order to reduce the length of the MMI shown in FIG. 1a, it is therefore necessary also to reduce the MMI width and also the gap 5 between the input/output waveguides, by reducing their spacing, S, shown in FIG. 1a. It has been demonstrated in many commonly used semiconductor etch processes, that the etched depth of the waveguide 12 is dependent on the etched gap 5 and also that the slope of the waveguide sidewall is not vertical, particularly at the base of the waveguide, and also depends upon the etched depth.
FIG. 1c shows schematically the variation of the etched depth as a function of the etched gap for the waveguides 12 shown in FIG. 1b. The etched depth of the waveguides 12 is shallower if the etching process produces a smaller etched gap 5. In such an arrangement, there is a risk that the etched depth does not fully penetrate through the waveguide core layer 2, for closely spaced waveguides.
FIG. 1d is a schematic cross-section of a pair of input ridge waveguides of a 2×2 MMI coupler in which the ridge etch depth is dependent upon the waveguide gap. As can be seen, the ridge profile is asymmetric and an inside wall 6 of each waveguide 12 is not vertical. The waveguides 12 are therefore individually left-right asymmetric. One of the effects of such an asymmetrical arrangement is polarisation rotation. In this case, the state of polarisation of the light propagating in each of the two input waveguides is rotated in opposite directions, and so will become unequal between the two waveguides. The disadvantage of this, for example, in the case that the 2×2 MMI is used as a recombiner in an MZ interferometer, is that the light from the two input waveguides will not interfere completely, leading to a degradation of the extinction ratio of the interferometer.
Furthermore, waveguides which do not have substantially the same profile/shape (at the input and output of the MMI) can lead to degradation of the performance of the coupler. In particular, the required imaging of the input optical modes to the desired output optical modes, which is achieved by means of the multi-mode optical interference behaviour of the coupler, is impaired if either the input or output waveguides are incorrectly positioned or are not matched. This impairment may take the form of increased optical loss (reduction in optical power), or in errors in the relative optical power or in the relative optical phase between the signals at each of the MMI waveguide outputs. In addition, when the input/output waveguides do not have the same ridge profile over a significant length leading to the MMI, their propagation characteristics are different. This could lead to imbalance in a MZ interferometer in which the waveguides are contained.
A possible solution to the polarisation rotation is to etch the waveguides to a deeper depth so as to ensure a vertical sidewall at the waveguide-core for closely-spaced waveguides. However, this solution is not possible in many practical cases, because of an upper limit for the maximum waveguide etched depth for widely spaced waveguides which may arise from other optical circuit design considerations.
Thus there is a need for a waveguide arrangement design which will address the disadvantages associated with etch-process induced asymmetries in the shape of closely spaced waveguides.